When airflow passes over a body there are two fundamental mechanisms that produce a drag force. These forces come from surface drag, caused by friction as the air passes over the surface, and pressure drag caused primarily by the separation of vortices from the boundary layer. The ratio of surface drag to pressure drag is highly dependent on the shape of the object. Where objects are specifically shaped for optimum aerodynamic efficiency, the aspect ratio (length: width) will generally be at least 3:1. With an increased length to width ratio it is possible to have a wing-like shape with a narrow trailing edge. The advantage of this is that the flow can remain attached to the surface of the object so that the streamlines follow the shape of the profile. Although the surface area of the object and the resulting surface friction are increased, the flow is able to “recover” beyond the widest point of the object, resulting in a small net pressure drag. Generally, the reduction in pressure drag far outweighs the increase in surface drag.
The human body tends to have a much lower aspect ratio, particularly when upright, which may typically be nearer to 1:1 for the arms and legs, and 1:2 for the torso. As a result, the human body approximates to a “bluff body”, and pressure drag tends to be by far the larger contributory factor to the overall aerodynamic drag experienced by an athlete.
Where it is not practical to modify the shape of the body and the aspect ratio is lower than about 3:1 in the flow direction, a high level of pressure drag can be caused by flow separation soon after the flow has passed the widest point of the body. In such situations in engineering and nature, it is known to adjust the surface texture of the body to help delay the separation point and thereby reduce the net pressure force that retards motion of the object.
A number of techniques are known to reduce the net drag force on bluff bodies, including the use of trip edges and textured surfaces. Although these techniques may give rise to an increase in surface drag, it is generally possible to find a solution whereby the reduction in pressure drag outweighs the increase in surface drag. This allows the total drag to be reduced in various applications. However, current technologies have the following limitations:                Trip edges can be very effective in ideal circumstances, but in practice they are extremely sensitive to position. If the trip edges are not placed precisely in the correct locations they can have a detrimental effect, increasing the overall drag. This means that trip edges, or multiple trip edges, are not appropriate for commercial clothing applications, where the exact shape of the body is unknown.        Environmental conditions can affect the onset of turbulent flow within the system in which the subject is positioned, and are variable and unpredictable. For example, the flow direction experienced by a cyclist can vary by 10° or more from the direction of travel owing to crosswind effects. Experience has shown that it is not possible to have a trip edge that works effectively for all conditions.        Textured surfaces work to an extent, but the types of textured surfaces available are limited and they are often designed for purposes that are not specific to delaying flow separation.        Fabrics with different textures are sometimes used in sports clothing and in certain circumstances this can reduce drag. However, changes in fabric texture often require the presence of seams, which can have a detrimental effect on the overall drag. Also, fabrics tend to be provided with uniform repeating texture patterns, which are not optimised to control flow separation.        
The ideal surface roughness is heavily dependent on a number of factors, including forward velocity and body shape (curvature and body length), and ideally needs to change constantly along the flow direction to introduce perturbations into the flow that aid flow attachment, whilst not significantly increasing the surface drag. The optimum texture needs to change constantly to provide the correct height and level of disturbance for the air passing over a given point within the boundary layer. Currently, no textile products are available that can offer an optimum level of performance for a given application.